The virus Zafi-D, which first appeared on the Internet at the end of last year, led a monthly list of top 10 viruses for the fifth straight month in April, a security firm said Friday.

Zafi-D accounted for 46.6 percent of all virus reports submitted to Sophos, based in England. Only one new threat, Mytob-Z, broke into the top 10 in April, making its debut at No. 7.

"During the month of April, several older viruses maintained their spot on the chart," Gregg Mastoras, senior security analyst at Sophos, said in a statement. "The Zafi family of viruses accounted for more than 50 percent of all the viruses reported to Sophos this month. It could be that many of these worms were able to spread in multiple languages, therefore catching some users off guard."

Most viruses are sent as attachments to emails that use a variety of schemes to get PC users to open the malicious files. Many viruses open a backdoor in a PC that enables hackers to control the computers to gather personal information, send spam or launch denial of service attacks.